Barrier stretch film particularly for the packaging of food products and method for the production thereof

ABSTRACT

A stretch film presenting controlled permeability to gases, manufactured by process of balloon co-extrusion and consisting of outer layers (S 2 )of stretchable polymeric material and by one or two inner barrier layers (S 3 ) of a EVOH based polymeric matrix, which is made stretchable by the addition of a suitable plasticizer in such a percentage as to conciliate the stretchability with the desired barrier effect, said layers of different material being reciprocally secured by means of layers (S 4 ) of adhesive material. The polymeric matrix which forms the barrier material and the plasticizer are intimately mixed inside a double-screw corotating extruder, immediately before the phase of co-extrusion, through a premelting of the polymeric matrix and the subsequent injection of the plasticizer in liquid phase.

DESCRIPTION

[0001] The present invention relates to stretch films particularly forthe packaging of food products and more particularly it relates to astretch film for the packaging of food products in a modified atmosphereand provided with a barrier presenting controlled permeability to gases,so as to ensure a time prolonged preservation of the packaged goods.

[0002] In the field of packaging of food products it is known, as in thecase of portioned fresh meat, the use of stretch film in order to obtaina protective wrapping, which represents simply a barrier to the surfacecontamination subsequent to the packaging and which is not capable ofpreventing the degradative development of the packaged product.

[0003] In order to prolong the preservation of the packaged products, bypreventing or limiting in a significant manner the microbic developmentand by partially rendering inactive the enzymatic processes, there isused the technology of packaging in protective atmosphere, by employingthe so-called barrier films, which are substantially impermeable togases. However, the known materials having the property of preventingthe permeation of gases, present also the characteristic of not beingstretchable, or of not being stretchable to a sufficient degree, whichfact precludes the flexibility of use which is typical of the stretchfilm employed for the packaging of food products.

[0004] According to document EP 0 932 550 there is contemplated the useof films presenting high stretchability and sealability for thepackaging of food products, so as to obtain closed packages with a tightseal, inside which there is created, at the moment of their formation atthe interior of a controlled atmosphere chamber, a modified atmospheresuch as to guarantee a preservation more accurate and prolonged in timeof the packaged product.

[0005] From the present state of the art there is known the barrierstretch film described in document EP 0 333 443, which comprises amulti-layer film formed by outer layers of heat sealable polymericmaterial, by a first pair of inner layers of an oxygen barrier polymericmaterial, by a second pair of inner layers of a high molecular weightpolymeric material, by a third pair of inner layers copolyamide based,and which comprises layers of polymeric adhesive between said second andsaid third pairs of inner layers, said multi-layer film being made as atubular film and its interior surface being self-sealing. For therealisation of the inner layers of polymeric material havingcharacteristics of a high gas barrier there are cited both copolymersethylene vinyl alcohol copolymer (EVOH) and polyvinylidene chloridecopolymer (PVDC).

[0006] According to said document there is made preference for the useof the PVDC manufactured as “Saran” by Solvay, due to the relative minorsensibility to humidity of this copolymer with respect to EVOH.

[0007] Particularly, for the uses for which this type of film isproposed, where a high gas barrier is requested, the employ of notplasticized Saran is suggested, since the possible plasticization,mentioned in generic terms, although it confers to the copolymer agreater flexibility, prejudices the desired characteristics of highbarrier to gases.

[0008] The present invention intends to obviate the limits of the knownart by proposing a multi-layer film particularly adapted for thepackaging of food products, which is able to conciliate the requirementof a high stretchability and of an Impermeability to gases, suitable forthe product to be packaged, and which is characterised by comprising atleast one barrier layer consisting of a polymeric matrix substantiallyimpermeable to gases, EVOH-based, which is rendered stretchable byadding a suitable plasticizing material, said barrier layer beingenclosed between at least two layers of a film of stretchable polymericmaterial. Differently from the PVDC of the known art, the plasticizedEVOH according to the present invention is provided with the requiredstretchability and at the same time it maintains a high barrier togases. Advantageously there is used a composite film provided with amultiple barrier layer, for example double, with the barrier layersunited between them and to the outer layers by means of layers ofadhesive material, in such a manner that the flexibility of themulti-layer structure thus obtained, is such that during the phase ofstretching of the film, the two barrier layers behave in a solidarymanner, substantially without reciprocal slips, giving origin to areciprocal compensation, guaranteeing uniformity of thickness of thestretched film and a gas barrier which is suitable for the mentionedscopes.

[0009] According to the present invention, the barrier layer consists ofEVOH with the addition of a plasticizer the percentage of which can varyfrom a minimum to a maximum value, for example from 5 to 30% dependingupon the packaging requirements. This possibility of variation of thepercentage of plasticizer consents the modulation of both thepermeability of the layer as. a function of the tightness which thepackaging film must confer to a determined food product, and the degreeof stretchability of the said barrier layer. For each product to bepackaged it will be therefore possible to select the combination ofpermeability/stretchability of the barrier film of films which is moreapt to guarantee the better conditions for the preservation of the saidproduct.

[0010] The outer layers of stretchable material and the adhesive layersare preferably polyolefinic based an/or other base such as to offer thebest protection against moisture to the layers of copolymer EVOHplasticized, so that even under high relative humidity conditions thereare not compromised the characteristics of high barrier to the gases ofthe multi-layer film according to the invention.

[0011] The multi-layer film according to the invention is preferablyproduced by means of a process of balloon co-extrusion of the layers ofstretch material, of barrier material and of adhesive material, inaccordance with the method described in document LU 88839 to form a filmof several layers glued the one to the other. Thanks to a correctbalance between the transverse drawing, caused by the balloon inflationand the longitudinal drawing, determined by the pull of the presserrolls which close the balloon, in relation with the gap of the extrusionhead, there are determined the orientation of the material and theperformances of the resulting film. Particularly, the gas barrierproperty is definitely fixed following to the drawing of the film in theproduct packaging machines, which causes an increase of the degree ofcrystallinity of the material, due to a molecular re-alignment, whichimplies a more orderly distribution with the consequent effect of areduction of permeability to gases of the film itself.

[0012] Further scopes and advantages of the present invention willappear better evident from the following description made with referenceto the attached drawings, in which:

[0013]FIG. 1 is a diagrammatic view of a plant for the manufacture ofthe multi-layer film according to the invention, provided with fivelayers comprehensive of a single barrier layer;

[0014]FIG. 1a shows diagrammatically details of the co-extrusion head ofthe plant of FIG. 1, sectioned along line l-l and connected to theinjections extruders of the various layers of materials;

[0015]FIG. 1b shows in transverse section the multi-layer film producedby the plant of FIG. 1;

[0016]FIG. 2 is a diagrammatic view of a plant for the manufacture ofthe multi-layer film according to the invention, provided with sevenlayers comprehensive of two barrier layers;

[0017]FIG. 2a shows diagrammatically details of the co-extrusion head ofthe plant of FIG. 2, sectioned along line II-II and connected to theinjections extruders of the various layers of materials;

[0018]FIG. 2b shows in transverse section the seven-layer film producedby the plant of FIG. 2.

[0019] Hereafter it is indicated by way of non-limiting example, thechemical specification of a barrier layer according to the invention,which layer was subjected to a test of permeability to oxygen underpredetermined conditions.

[0020] Chemical Specification of the Barrier Layer.

[0021] Polymeric Matrix: ethylene vinyl alcohol copolymer (EVOH) havingthe general formula:

[0022] in which “m” (ethylene contents)=27÷47 mol %.

[0023] Plasticizer: It is preferably selected in the class of the diolsand triols having low molecular weight, with the possible presence of anetheric bond. As an example of a suitable plasticizer there is citedpropylene glycol (1,2-propanediol) having as general formula:

C₃H₈O₂

[0024] Other plasticizers which can be used can be butanediol,propanetriol or glycerol or pentanediol.

[0025] The intimate mixing between EVOH and the plasticizer is performedinside an extruder of double-screw, co-rotating type, immediately beforethe co-extrusion process, through a pre-melting of the polymeric matrixEVOH and the consequent injection of the liquid plasticizer which, byhomogenous dispersion between the chains of the polymer, reduces in asensible manner its intermolecular strength.

[0026] The temperature profile during this phase is such as to avoid theevaporation of the plasticizer and, consequently, the plasticization ofthe polymeric matrix EVOH is exactly defined and such as to ensure thestability of the final product during time.

[0027] The temperature profile is correlated to the percentage of theplasticizer employed, and with a plasticization of ten percent withpropylene glycol, it is of the type indicated in the following table: T1T2 T3 T4 T5 T6 T7 T8 T9 T10 T11 T12 210 210 200 190 185 180 180 180 180180 180 180

[0028] The resulting product, although it maintains an excellent barrieraction to the gases, aromas and flavours, typical of the base polymerEVOH, acquires improved optical properties, gloss and transparency,together with a high stretchability.

[0029] From the calorimetric investigation it appears that the vitreoustransition temperature (Tg) decreases in a determining manner withrespect to the EVOH as such and this decrease, which is the moreremarkable, the higher is the percentage of plasticizer introduced,consents to the final film to be subjected to extensions of remarkableentity (up to 150%) at ordinary temperatures.

[0030] The possibility of modulating the percentage of plasticizer,preferably between 5 and 30% consents to obtain films characterised by adifferent permeability to gases (which is the lesser, as the lesser isthe degree of plasticization) together with a different stretchability(which is the higher as the higher is the degree of plasticization).

[0031] In the following table there are indicated some data ofpermeability to oxygen for a test piece consisting of the abovedescribed barrier layer, with a thickness of 20 microns and tested underthe conditions hereafter indicated:

[0032] T=23° C., 0% R.H. (ASTM D1434) TABLE 1 Tested test-piece referredto Permeability to 0₂ a thickness of 20 microns cc/m² × 24 h × atm. EVOHnot plasticized 2.5 EVOH plasticized at 5% with propylene glycol 4.0EVOH plasticized at 10% with propylene glycol 5.5 EVOH plasticized at15% with propylene glycol 7.0 EVOH plasticized at 20% with propyleneglycol 9.0

[0033] Based on these considerations it is possible to find the bestcombination of the two parameters (permeability to gases /stretchability) which better responds to the packaging requirements of adetermined product.

[0034] Concerning the structure of the stretch film with barrier effect,particularly for the packaging of food products according to the presentinvention, we give some non-limiting examples of realisation of saidstructures.

EXAMPLE 1

[0035] Structure with a Single Barrier Layer

[0036] In this particular structure we have a sequence of layersconstituting the packaging film diagrammatically listed hereafter: Outerlayer Adhesive Barrier layer Adhesive Inner layer

[0037] The outer and inner layer define respectively the outer face andthe inner face of the co-extruded balloon for the formation of themulti-layer film, which balloon is thereafter squeezed and then dividedin two parts which are collected separately (see after). It is to beunderstood that the layer previously defined as “inner”, actually is anouter layer inasmuch as it defines an outer face of the multi-layer filmaccording to the invention.

EXAMPLE 2

[0038] Structure with a Double Barrier Layer Outer Adhesive BarrierAdhesive Barrier Adhesive Outer layer layer layer layer

[0039] As in the preceding example, the outer layers represent the outerfaces of the co-extrusion balloon of the multi-layer film according tothe invention (see after), which is squeezed to form a single film whichis collected singularly.

[0040] Th uter and inner layrs of Example 1 and the outer lay rs ofExample 2 are made of materials presenting high stretchability andsealability, for example by materials having a polyolefin base.

[0041] Adhesive layers: The adhesive layers are made for example ofmaterials having a polyolefin base, modified by the addition offunctional polar groups, such as maleic anhydride.

[0042] The structure presenting a double barrier layer is the one whichis better suitable for solving all the technical problems connected withthe present invention. The materials which compose the outer stretchablelayers and the adhesive layers, are as mentioned of a polyolefin base epresent a high impermeability to the water vapour, so that they offerthe best protection against humidity to the EVOH based barrier layers,so that even in the presence of high values of relative humidity, thecharacteristics of high barrier to the gases of the multi-layer filmaccording to the invention are not compromised.

[0043] With reference to FIGS. 1, 1a, 1 b, 2, 2 a, 2 b of the twoattached sheets of drawings, the process is now described for theproduction on industrial scale of the stretchable and barriermulti-layer film according to the above mentioned Examples 1 and 2,which foresees the use of the machine described in document LU 88839.

[0044] In FIGS. 1 and 1a, reference numeral 1 indicates a co-extrusiondie head to which there are fed by three extruders indicated byreferences E2, E3 and E4 respectively the inner and outer layer ofstretchable material, the intermediate layer of barrier material and thetwo layers of adhesive material, arranged between the two stretchablelayers and the barrier layers in order to unite them intimately. Thetubular balloon 301 which comes out of the co-extrusion head 1 and whichis formed by the five layers of above mentioned materials is handled bymanipulator means mounted on a frame 3 which rotates slowly first in onedirection and then in the other direction, about an axis horizontal andaligned to the axis of the said head, and which means comprise insuccession:

[0045] a first driven pair of parallel drawing rollers 4, 104 whichsqueeze and render flat the said film balloon 301, so as to form twosuperposed films F1 and F1′, each one comprising five layers as from theabove said Example 1, which are in reciprocal contact by the inner layerof stretchable film;

[0046] cutting means 5 which separate from the two superposed films therelating longitudinal connecting edges. Suitable means not shown areprovided for discarding the off-cuts resulting from the cuttingoperation;

[0047] a second driven pair of parallel rollers 6, 106 which operate ata speed which is suitable greater than the one of the said first pair ofdrawing rollers, in order to subject the said films to a suitablelongitudinal stretch and to maintain them in the best condition ofco-operation with said cutting means;

[0048] winding means for the winding into bobbins of the five-layerfilms F1 and F1′.

[0049]FIG. 1b shows in section one of the five-layer films F1, F1′produced by the process according to FIG. 1 and 1 a, and comprises outerlayers S2 of stretchable material, an inner layer S3 of barrier materialand intermediate layers S4 of adhesive material.

[0050] In FIGS. 2, 2a there is instead illustrated the process for theproduction of the seven-layer film in accordance with Example 2. Thethree extruders E2, E3 and E4 as in the preceding embodiment, feed tothe co-extrusion head 1′ respectively the outer layer of stretchablematerial, the intermediate layer of barrier material and the two layersof adhesive material arranged respectively between the two abovementioned layers and at the interior, so as to form the inner wall ofthe balloon 301′ which comes out of the same head. When the balloon 301′passes through the first pair of parallel drawing rollers 4, 104, It issqueezed and thanks to the presence of the inner layer of adhesivematerial, the two portions of the said balloon adhere intimately the oneto the other so as to form a single film having seven layers and adouble barrier layer, with outer layers of stretchable material as fromthe above said Example 2. The film is further stretched by the secondpair of drawing rollers 6,106, it is deprived of the longitudinal edgesby the cutting means 5 and is finally collected onto a single bobbin 7.From FIG. 2b it appears that film F is composed of outer layers S2 ofstretchable material, of two inner layers S3 of barrier material and ofthree intermediate layers S4 of adhesive material, of which the centrallayer is double.

[0051] It is to be understood the description was referred to somepreferred embodiments of the invention, to which there can be made manyvariations and modifications, particularly in the construction, thewhole however without departing from the leading principle of theinvention, as above described, illustrated and claimed hereinafter.

1. A stretch film particularly for the packaging of food products,presenting controlled permeability to gases, characterised by comprisingat least two outer layers (S2) made of polymeric stretch material withinterposed at least one barrier layer (S3) made of a polymeric matrixsubstantially impermeable to gases, EVOH based, with the generalformula:

in which “m” (ethylene contents)=27÷47 mol %, said matrix being madestretchable by the addition of a suitable plasticizer material, in sucha percentage as to conciliate the stretchability with the desiredbarrier effect, the barrier layer being united to the said outer layersof stretchable material by means of layers (s4) of adhesive material. 2.A multi-layer film according to claim 1, characterised by the fact thatat least the outer layers of stretchable material (S2) and preferablyalso the layers (S4) of adhesive material have polymeric bases withcharacteristics of high impermeability to water vapour, so as to protectthe EVOH based barrier layer or layers (S3), so that even underconditions of high relative humidity, the multi-layer barrier accordingto the invention maintains unaltered its characteristics of barrier togases.
 3. A multi-layer film according to claim 2, in which the outerlayers (S2) and the adhesive layers (S4) consist of polyolefinic basematerials.
 4. A multi-layer film according to claim 2, in which theadhesive layers (S4) consist of polyolefinic base materials with theaddition of polar functional groups, such as maleic anhydride.
 5. Amulti-layer film according to claim 1, in which the percentage ofplasticizer material added to the EVOH polymeric matrix of the barrierlayer (S3) varies from a minimum value of about 5% to a maximum value ofabout 30/o.
 6. A multi-layer film according to the preceding claims, inwhich the plasticizer material is elected in the class of the diols orof the triols having low molecular weight, with the possible presence ofan etheric bond.
 7. A multi-layer film according to claim 6, in whichthe plasticizer material is propanediol.
 8. A multi-layer film accordingto claim 6, in which the plasticizer material is butanediol.
 9. Amulti-layer film according to claim 6, in which the plasticizer materialis pentanediol.
 10. A multi-layer film according to claim 6, in whichthe plasticizer material is propanetriol or glycerol.
 11. A multi-layerfilm according to claim 1, characterised by consisting of a structurecomprising five layers which sequentially comprise: an outer layer (S2)of stretchable material-base, an adhesive layer (S4), a plasticizedbarrier layer (S3), an adhesive layer (S4) and an outer layer (S2) ofstretchable material base.
 12. A multi-layer film according to claim 11,characterised by the fact that said five-layer structure is obtained bymeans of balloon co-extrusion of the stretchable material, of theplasticized barrier material and of the adhesive material, withsubsequent separation of the two flattened and superposed portions ofthe balloon itself, and by means of separate collection of the twomulti-layer films (F1, F1′) thus obtained.
 13. A multi-layer filmaccording to claim 1, characterised by consisting of a structurecomprising seven layers which sequentially comprise: an outer layer (S2)of stretchable material base, an adhesive layer (S4), a firstplasticized barrier layer (S3), an adhesive layer (S4), a secondplasticized barrier layer (S3), an adhesive layer (S4) and an outerlayer (S2) of stretchable material base.
 14. A multi-layer filmaccording to claim 13, characterised by the fact that said seven-layerstructure is obtained by means of balloon co-extrusion of thestretchable material, of the plasticized barrier material and of theadhesive material, the two flattened and superposed portions of theballoon itself being secured the one to the other due to the presence ofadhesive material, so as to form a single film (F) which is collectedinto a single bobbin.
 15. A process for the manufacture of a multi-layerfilm which is stretchable and presents a controlled permeability togases, according to claim 1, characterised by comprising: a phase ofballoon co-extrusion of the stretchable material, of the plasticizedbarrier material and of the adhesive material fed by respectiveextruders, said layers being arranged so as to form an outer annularlayer and an inner annular layer of stretchable material, anintermediate annular layer of plasticized barrier material and annularlayers of adhesive material interposed between the preceding layers;squeezing and longitudinal stretching of the balloon by means of a firstand of a second pair of parallel and superposed rollers; cutting anddiscarding the two longitudinal edges which reciprocally connect thesuperposed portions of the balloon so as to obtain two films (F1, F1′)which can be separated, this phase being performed on the film tensionedbetween the said two pairs of drawing rollers; separate collection ofthe two five-layer films obtained in the preceding phase; the phases oflongitudinal stretch, of longitudinal cutting and of collection of thefilm being obtained on a structure (3) capable of alternately rotatingof about 360° around the axis of the balloon.
 16. A process for themanufacture of a multi-layer film which is stretchable and presents acontrolled permeability to gases, according to claim 1, characterised bycomprising: a phase of balloon co-extrusion of the stretchable material,of the plasticized barrier material and of the adhesive material fed byrespective extruders, said layers being arranged so as to form an outerannular layer of stretchable material, an intermediate annular layer ofplasticized barrier material and two annular layers of adhesive materialthe first one internal and the other one interposed between the twopreceding layers; squeezing and longitudinal stretching of the balloonby means of a first and of a second pair of parallel and superposedrollers, so as to obtain a seven-layer film; cutting and discarding ofthe two longitudinal edges of the obtained seven-layer film, this phasebeing performed on the film tensioned between the said two pairs ofdrawing rollers; collection of the seven-layer film; the phases oflongitudinal stretch, of longitudinal cutting and of collection of thefilm being obtained on a structure (3) capable of alternately rotatingof about 360° around the axis of the balloon.
 17. Process according toclaim 15, in which the polymeric matrix which constitutes the barriermaterial and the plasticizer are intimately mixed inside a double-screwco-rotating extruder immediately before the phase of co-extrusion,through a pre-melting of the polymeric matrix and the subsequentinjection of the plasticizer in liquid phase.
 18. Process according toclaim 16, in which the polymeric matrix which constitutes the barriermaterial and the plasticizer are intimately mixed inside a double-screwco-rotating extruder immediately before the phase of co-extrusion,through a pre-melting of the polymeric matrix and the subsequentinjection of the plasticizer in liquid phase
 19. Process according toclaim 17, in which the profile of the plasticization temperature of thepolymeric matrix which constitutes the barrier material varies as afunction of the percentage of added plasticizer and the injection of theplasticizer takes place at temperature levels which are lower than therelative evaporation temperature.
 20. Process according to claim 18, inwhich the profile of the plasticization temperature of the polymericmatrix which constitutes the barrier material varies as a function ofthe percentage of added plasticizer and the injection of the plasticizertakes place at temperature levels which are lower than the relativeevaporation temperature.
 21. Process according to claim 19,characterised by the fact that if the EVOH polymeric matrix isplasticized at 10% with propylene glycol, the pre-melting of the matrixstarts at about 210° C., the injection of the plasticizer is effected atabout
 22. Process according to claim 20, characterised by the fact thatif the EVOH polymeric matrix is plasticized at 10% with propyleneglycol, the pre-melting of the matrix starts at about 210° C., theinjection of the plasticizer is effected at about 180° C.